Representatives from Canada’s trucking industry warned Canadian senators that any increase in border congestion from pot legalization could slow down cross-border trade and hit the economy – even if that’s just from slower passenger lanes at customs.

“Anything that negatively impacts, slows down or restricts access … will flow through to the wider Canadian economy,” said Jonathan Blackham, the director of policy and public affairs for the Canadian Trucking Alliance.

If that happens in passenger lanes after legalization, he said that would still “flow through in an impact to us, just due to the physical nature of the border itself.”

That said, it’s a “great unknown” whether or not that will happen once recreational cannabis is legalized – or even by how much. But Blackham said there needs to be policy in place to mitigate potential backlogs.

U.S. border guards can question anyone on whether or not they’ve consumed cannabis recreationally, and then turn them back. Although, for commercial vehicles, Blackham said it will be business as usual after the bill is passed; they already fall under tight regulations and commercial vehicle drivers shouldn’t consume cannabis at all.

Blackham was one of a number of witnesses testifying at the Senate’s National Security and Defence committee Monday on how legalizing recreational cannabis could affect Canada-U.S. border operations.

Windsor Mayor Drew Dilkens said “nothing” is very clear about what happens after cannabis legalization, although it will undoubtedly provoke a heightened interest from Americans – not to mention, spark ‘cannabis tourism’.

While he said he doesn’t think that “Rome is going to burn” and that borders will clog up after the bill is passed, he suggested there could be a short-to-medium-term thickening of the border where officials on both sides adapt to the change.

Windsor is a major trade and travel port of entry to the U.S., and Dilkens said a “significant” amount of money needs to be spent on border education efforts about legal cannabis.

At the committee’s last meeting, Public Safety Minister Ralph Goodale said traffic congestion has been the number one concern raised by the U.S. about legalization.

He said he’s told the U.S. the rules aren’t changing at the border, so officials shouldn’t press about cannabis consumption at customs because it’s irrelevant to border security today.

“Our message to the Americans is this should not be an issue,” he said. “It becomes an issue if you make it one, and there’s no need to make it one because the border rules have not changed.”

While it’s not necessary for customs officials to ask about cannabis consumption, various witnesses have suggested that experience has shown many people have been turned away at the border with lifetime bans.

Some have predicted that will only increase once pot is legalized.

Scott Railton, a U.S. immigration lawyer at Cascadia Cross-Border Law, said don’t bet on U.S. customs relinquishing its authority to ask about pot consumption; but it might soften its policy by issuing guidelines like it has done with searching electronic devices.

Still, he said there will likely be some “growing pains if this bill goes forward.”

The Senate is currently studying Bill C-45, which will legalize and regulate recreational consumption of cannabis in Canada.

The Trudeau government has said it wants to see cannabis legalized by this summer, but the federal Conservatives have opposed the bill and called for its delay. Some Tory senators have said they want to see it scrapped or significantly re-written.

The Senate is expected to vote on the bill at third reading stage on or before June 7.